The ability to rapidly detect and analyze the concentration of a variety of ionic species or analytes in solution is helpful in many settings, for example, in the clinical, analytical or industrial chemical laboratory, or in large scale industrial or municipal applications such as water and wastewater treatment. Conventionally, in such analyses, an ion selective electrode (ISE) is brought into contact with a test solution into which a reference electrode is also immersed. The ion selective electrode and the reference electrode are connected via a voltmeter and a potentiometric determination of the activity of a particular analyte in solution is made. The activity measurement may be correlated to the concentration of the analyte in solution using reference solutions or standard solutions of known concentration.
Ion-selective ionophores are known, such as ionophores for ammonium, nitrate, nitrite lithium, sodium, potassium and calcium ions. It is known that for ISE electrode stability an internal electrochemical reference is needed. U.S. Pat. No. 4,431,508 discloses that a solid state ISE probe with a sylonized graphite rod reduces the drift of the probe. U.S. Pat. No. 4,214,968, U.S. Pat. No. 5,286,365 and U.S. Pat. No. 5,840,168 disclose that using an internal electrochemical reference element, usually as a Metal/Metal Salt composition in equimolar concentration, increases the stability of the ISE signal. However, known ion selective electrode sensors continue to suffer from problems with ISE signal drift and often involve very complicated production of compositions for the internal electrochemical reference. A solid state ISE with an internal electrochemical reference element causes the signal to drift over time in an unpredictable fashion that makes compensation difficult.